Organic compounds include more varied material systems in comparison with inorganic compounds, and have possibility that materials of various functions can be synthesized by an appropriate molecular design. Also they have features that a molded article such as a film is flexible and provides an excellent workability by a polymer formation. Based on these advantages, photonics and electronics utilizing functional organic materials are attracting attention recently.
Examples of an electronic device utilizing an organic compound material as a functional organic material include a solar cell, an electric field light emitting device, and an organic transistor. These are devices utilizing electrophysical properties (carrier transporting property) and optophysical properties (light absorption or emission) of the organic compound material, and, among these, the electric field light emitting device is showing a remarkable progress.
As a most basic device structure of the electric field light emitting device, there is known a structure in which a thin film of a total thickness of about 100 nm, formed by a hole transporting layer constituted of a hole transporting organic compound and an electron transporting light-emitting layer constituted of an electron transporting organic compound, is sandwiched between electrodes (for example cf. non-patent reference 1). By applying a voltage on such device, a light emission can be obtained from the electron transporting organic compound which also has a light emitting property. Such structure is generally called a single hetero (SH) structure.
The electric field light emitting device in the non-patent reference 1 can be considered to be based on a functional separation, namely the transportation of holes being executed by the hole transport layer and the transportation of electrons being executed by the electron transport layer.
Thereafter, for the purposes of further improvement in a change of an emission spectrum and a decrease in the light emission efficiency resulting from an interaction (for example formation of an exciplex) at an interface of the laminated layer, the concept of such functional separation has been developed into a double hetero (DH) structure in which a light emitting layer is inserted between the hole transport layer and the electron transport layer (for example cf. non-patent reference 2).
In an electric field light emitting device as described in the non-patent reference 2, in order to further suppress an interaction generated at the interface, it is preferable to form the light emitting layer with a bipolar material having both the electron transporting property and the hole transporting property.
However, organic compound materials are mostly a monopolar material, having either the hole transporting property or the electron transporting property. For example, a material shown in the following patent reference 1 is only applied as an electron injecting layer.
It is therefore desired to newly develop an organic compound material having a bipolar character.
Patent reference 1: JP-A-2003-40873
Non-patent reference 1: C. W. Tang and one another, Applied Physics Letters, Vol. 51, No. 12, 913-915 (1987)
Non-patent reference 2: Chihaya Adachi and three others, Japanese Journal of Applied Physics, Vol. 27, No. 2, L269-L271 (1988).